The invention relates to transmitting data between end stations, one of which is referred to as a "transmit end station", the other being a "receive end station". The invention is applicable in particular, but not exclusively, to transmitting data by radio.
More precisely, the invention relates to backing up such data transmission by using at least one back-up channel in addition to N normal channels, where N.gtoreq.1. Usually, one back-up channel is used for a plurality of normal channels.
It is necessary to make data transmission reliable whenever transmission over the channels might be impaired, such impairment giving rise to transmission errors. For example, in the case of data transmission by radio, transmission over the channels might be impaired because of atmospheric conditions, or because of reflections or fading. In general, transmission is made reliable by using a back-up channel instead of a normal channel (generally, the normal channel that is most impaired).
In this type of data transmission over channels, the data transmitted over each of the N normal channels, and the data transmitted over the back-up channel, generally comprises both traffic data and supervisory data.
Traffic data (also referred to as "payload" data) may, for example, be supplied to the transmit end station in the form of N incoming payload paths. For example, each incoming payload path may have a data rate of 34 Mbit/s.
The supervisory data (also referred to as "service" data) is in particular data relating to the various protocols implemented for the transmission. For example, it is supplied to the transmit end station in the form of N incoming service paths.
Each of the N incoming service paths is associated with a respective one of the N incoming payload paths. The transmit end station thus generally includes N pairs of inputs, each of which comprises a traffic data input and a supervisory data input. The traffic data and the supervisory data applied to the two inputs of the same pair of inputs is transmitted by the transmit end station over one of the N normal channels, and optionally over the back-up channel. In other words, the data from an incoming service path is multiplexed with the data from the associated incoming payload path so as to be transmitted in the same channel in the form of a stream of data frames.
Likewise, the receive end station also generally includes N pairs of outputs, each of which comprises a traffic data output and a supervisory data output. The data available at the respective outputs of the same pair of outputs is traffic data and supervisory data received by the receive end station over the same normal channel, and optionally over the back-up channel.
In the state of the art, for all data transmission systems of the above-described type, making data transmission reliable consists in performing the following successive steps:
in the transmit end station: PA1 in the receive end station: PA1 return to baseband (generally by aligning); PA1 demultiplexing the streams of frames, a payload path (for traffic data) and a service path (for supervisory data) being multiplexed on each of said streams; PA1 separate and error-free interchange of the demultiplexed payload paths and service paths; PA1 aligning the payload paths and the service paths; and PA1 multiplexing the aligned payload paths and service paths so that they can be transmitted. PA1 said method being characterized in that the following successive steps are performed in said transmit end station: PA1 and in that the following successive steps are performed in the receive end station: PA1 in which each relay station receives data over N incoming normal channel portions, and over at least one incoming back-up channel portion of a preceding transmission leg, and re-transmits the received data over N outgoing normal channel portions and over at least one outgoing back-up channel portion of a following transmission leg; PA1 the method is characterized in that the following successive steps are performed in said relay station: PA1 multiplexing means for multiplexing the traffic data and the supervisory data applied to each of the N pairs of inputs on a respective one of the N normal channels; and PA1 switching means for switching the multiplexed data from a "backed-up" one of the N normal channels to the back-up channel. PA1 interchange means for interchanging multiplexed data from the back-up channel with multiplexed data from a "backed-up" one of the N normal channels; and PA1 demultiplexing means for demultiplexing traffic data and supervisory data from each of the N normal channels, so as to make the data available on a respective one of said N pairs of outputs. PA1 interchange means for interchanging the multiplexed data from the incoming back-up channel portion with the multiplexed data from one of the N incoming normal channel portions, so as to obtain said N outgoing normal channel portions; and PA1 switching means for switching multiplexed data from one of the N outgoing normal channel portions to the outgoing back-up channel portion.
the traffic data is switched from a "backed-up" one of the N normal channels to the back-up channel; then PA2 supervisory data is multiplexed with the traffic data on each of the N normal channels; PA2 the traffic data and the supervisory data are demultiplexed on each of the N normal channels and on the back-up channel; then PA2 the traffic data from the back-up channel is interchanged with the traffic data from the backed-up normal channel. PA2 the traffic data and the supervisory data applied to each of the N pairs of inputs is multiplexed on a respective one of the N normal channels; and PA2 the multiplexed data from a "backed-up" one of the N normal channels is switched to the back-up channel; PA2 the multiplexed data from the back-up channel is interchanged with the multiplexed data from the backed-up normal channel; and PA2 the traffic data and the supervisory data on each of the N normal channels is demultiplexed so as to make it available at a respective one of said N pairs of outputs. PA2 the multiplexed data from the incoming back-up channel portion is interchanged with the multiplexed data from a "backed-up" one of the N incoming normal channel portions so as to obtain said N outgoing normal channel portions; and PA2 the multiplexed data from a "backed-up" one of the N outgoing normal channel portions is switched to the outgoing back-up channel portion.
That known technique of making data transmission reliable suffers from the drawback of enabling only traffic data to be made reliable, since the data is switched to the back-up channel prior to multiplexing the traffic data with the supervisory data. As a result, to make the supervisory data reliable, it is necessary to implement additional means which are complex and costly.
Furthermore, when the distance between the transmit end station and the receive end station is too great, one or more receive/retransmit relay stations are used. The reliable data transmission then has more than one transmission leg. Each transmission leg is implemented over N normal channel portions, where N.gtoreq.1, and over at least one back-up channel portion.
Thus, in the case of radio transmission, each transmission leg corresponds, for example, to a distance of about 50 km. A relay station is then placed every 50 km between the transmit end station and the receive end station.
In the case of such multi-leg transmission, it is always the above-mentioned technique for making data transmission reliable that is implemented in the transmit end station and in the receive end station.
A first known technique for making data transmission reliable, referred to as "full-link interchange" consists in interchanging a normal channel with the back-up channel over the entire transmission link connecting the transmit end station to the receive end station. The entire transmission link, i.e. all of the legs forming said link, is then made reliable as a whole. In other words, all of the successive back-up channel portions (which correspond to the successive legs of the link) serve to back up all of the successive portions of the same normal channel. In this case, the relay stations are used solely for regenerating the multiplexed supervisory data and traffic data in each channel, and optionally for inserting or dropping supervisory data.
That first known technique is simple to implement. Unfortunately, it leads to under-use of multi-leg transmission. Even if a single portion (i.e. a single leg) of a normal channel is impaired, all of the portions of said normal channel are nevertheless backed up. In other words, certain portions of the back-up channel are used to back up portions of a normal channel that are not impaired, whereas portions of another normal channel might be impaired but are not backed up.
In order to make optimum use of multi-leg transmission, it is necessary to back up the legs independently from one another. For this purpose, a second known technique for making data transmission reliable, referred to as "leg-by-leg interchange" consists in interchanging that normal channel portion which is most impaired on each leg with the back-up channel of said leg.
The advantage of a link with leg-by-leg interchange is that it offers better availability. Unfortunately, implementing it in all of the relay stations involves a large number of processing operations and therefore requires a large number of cards.
In the state of the art, there are two known manners of implementing such leg-by-leg interchange.
The first known manner consists in using two terminals back-to-back in each relay station with baseband interchange. That first solution, generally implemented in transmission of the N+1 type (N normal channels and 1 back-up channel) suffers from the drawback of making the following necessary in each relay station:
The second known manner of implementing leg-by-leg interchange consists in using "diversity" cards which interchange streams of frames. The second solution, generally implemented in transmission of the 1+1 type, suffers from the drawback of being difficult to apply to transmission of the N'+1 type, where N'&gt;1. The two streams of frames to be interchanged in each relay station (i.e. that from the normal channel and that from the back-up channel) are aligned using different clocks and independent time bases. In other words, the normal channel and the back-up channel are subjected to respective quasi-synchronous and independent treatments. As a result, the interchange performed in each relay station gives rise to errors, or even synchronization loss.